Typical power cables generally have one or more conductors in a core that is surrounded by several layers that can include: a first polymeric semiconducting shield layer, a polymeric insulating layer, a second polymeric semiconducting shield layer, a metallic tape shield and a polymeric jacket.
Polymeric materials have been utilized in the past as electrical insulating and semiconducting shield materials for power cables. In services or products requiring long-term performance of an electrical cable, such polymeric materials, in addition to having suitable dielectric properties, must be durable. For example, polymeric insulation utilized in building wire, electrical motor or machinery power wires, or underground power transmitting cables, must be durable for safety and economic necessities and practicalities.
One major type of failure that polymeric power cable insulation can undergo is the phenomenon known as treeing. Treeing generally progresses through a dielectric section under electrical stress so that, if visible, its path looks something like a tree. Treeing may occur and progress slowly by periodic partial discharge. It may also occur slowly in the presence of moisture without any partial discharge, or it may occur rapidly as the result of an impulse voltage. Trees may form at the site of a high electrical stress such as contaminants or voids in the body of the insulation-semiconductive screen interface. In solid organic dielectrics, treeing is the most likely mechanism of electrical failures which do not occur catastrophically, but rather appear to be the result of a more lengthy process. In the past, extending the service life of polymeric insulation has been achieved by modifying the polymeric materials by blending, grafting, or copolymerization of silane-based molecules or other additives so that either trees are initiated only at higher voltages than usual or grow more slowly once initiated.
There are two kinds of treeing known as electrical treeing and water treeing. Electrical treeing results from internal electrical discharges that decompose the dielectric. High voltage impulses can produce electrical trees. The damage, which results from the application of high alternating current voltages to the electrode/insulation interfaces, which can contain imperfections, is commercially significant. In this case, very high, localized stress gradients can exist and with sufficient time can lead to initiation and growth of trees. An example of this is a high voltage power cable or connector with a rough interface between the conductor or conductor shield and the primary insulator. The failure mechanism involves actual breakdown of the modular structure of the dielectric material, perhaps by electron bombardment. In the past much of the art has been concerned with the inhibition of electrical trees.
In contrast to electrical treeing, which results from internal electrical discharges that decompose the dielectric, water treeing is the deterioration of a solid dielectric material, which is simultaneously exposed to liquid or vapor and an electric field. Buried power cables are especially vulnerable to water treeing. Water trees initiate from sites of high electrical stress such as rough interfaces, protruding conductive points, voids, or imbedded contaminants, but at lower voltages than that required for electrical trees. In contrast to electrical trees, water trees have the following distinguishing characteristics; (a) the presence of water is essential for their growth; (b) no partial discharge is normally detected during their growth; (c) they can grow for years before reaching a size that may contribute to a breakdown; (d) although slow growing, they are initiated and grow in much lower electrical fields than those required for the development of electrical trees.
Electrical insulation applications are generally divided into low voltage insulation (less than 1 K volts), medium voltage insulation (ranging from 1 K volts to 69 K volts), and high voltage insulation (above 69 K volts). In low voltage applications, for example, electrical cables and applications in the automotive industry treeing is generally not a pervasive problem. For medium-voltage applications, electrical treeing is generally not a pervasive problem and is far less common than water treeing, which frequently is a problem. The most common polymeric insulators are made from either polyethylene homopolymers or ethylene-propylene elastomers, otherwise known as ethylene-propylene-rubber (EPR) or ethylene-propylene-diene ter-polymer (EPDM).
Polyethylene is generally used neat (without a filler) as an electrical insulation material. Polyethylenes have very good dielectric properties, especially dielectric constants and power factors. The dielectric constant of polyethylene is in the range of about 2.2 to 2.3. The power factor, which is a function of electrical energy dissipated and lost and should be as low as possible, is around 0.0002 at room temperature, a very desirable value. The mechanical properties of polyethylene polymers are also adequate for utilization in many applications as medium-voltage insulation, although they are prone to deformation at high temperatures. However, polyethylene homopolymers are very prone to water treeing, especially toward the upper end of the medium-voltage range.
There have been attempts to make polyethylene-based polymers that would have long-term electrical stability. For example, when dicumyl peroxide is used as a crosslinking agent for polyethylene, the peroxide residue functions as a tree inhibitor for some time after curing. However, these residues are eventually lost at most temperatures where electrical power cable is used. U.S. Pat. No. 4,144,202 issued Mar. 13, 1979 to Ashcraft, et al. discloses the incorporation into polyethylenes of at least one epoxy containing organo-silane as a treeing inhibitor. However, a need still exists for a polymeric insulator having improved treeing resistance over such silane containing polyethylenes.
Unlike polyethylene, which can be utilized neat, the other common medium-voltage insulator, EPR, typically contains a high level of filler in order to resist treeing. When utilized as a medium-voltage insulator, EPR will generally contain about 20 to about 50 weight percent filler, most likely calcined clay, and is preferably crosslinked with peroxides. The presence of the filler gives EPR a high resistance against the propagation of trees. EPR also has mechanical properties, which are superior to polyethylene at elevated temperatures. EPR is also much more flexible than polyethylene which can be an advantage for tight space or difficult installation.
Unfortunately, while the fillers utilized in EPR may help prevent treeing, the filled EPR will generally have poor dielectric properties, i.e. a poor dielectric constant and a poor power factor. The dielectric constant of filled EPR is in the range of about 2.3 to about 2.8. Its power factor is on the order of about 0.002 to about 0.005 at room temperature, which is approximately an order of magnitude worse than polyethylene.
Thus, both polyethylenes and EPR have serious limitations as an electrical insulator in cable applications. Although polyethylene polymers have good electric properties, they have poor water tree resistance. While filled EPR has good treeing resistance and good mechanical properties, it has dielectric properties inferior to polyethylene polymers.
Hindered amine light stabilizers or “HAL”s are primarily used in clear plastic film, sheets or coatings to prevent degradation by light. HALs are used in unfilled polyethylene insulations. They are thought to prevent degradation caused by light emitted by tiny electrical discharges. U.S. Pat. No. 5,719,218 discloses an optically transparent polyethylene insulation formulation with a HALs where it is stated that the HALs are useful for the prevention of degradation of the insulation by water trees.
U.S. Pat. No. 4,302,849 to Kawasaki et al proposes the use of high molecular weight polyethylene glycol as a solution to electrical insulation deterioration in polyolefin polymers. This technology has become widely used in the electrical cable industry, however, it is over 25 years old and the need for more improved performance in additives for treeing resistance exists.
Numerous methods to improve the performance of cross linked polyethylene (XLPE) insulation against dielectric deterioration by water tree generation and growth have been described in the literature. U.S. Pat. No. 4,144,202 issued Mar. 13, 1979, to Ashcraft et al relates to the inhibition of water tree growth by use of certain organosilane compounds. U.S. Pat. No. 4,206,260 describes compositions containing an effective mount of an alcohol containing 6-24 carbon atoms as being an efficient water and electrical tree retardant insulation. German patent 2,737,430 discloses that certain alkoxysilanes act as tree retardant additives in polyethylene insulation. European patent 0,166,781, published Jan. 8, 1986 to Sumitomo Electric Industries Limited describes a blend of ethylene and vinyl acetate copolymer as a water tree retardant material. Certain aliphatic carboxylic acid derivatives when incorporated in suitable mounts in XLPE are also reported to suppress water tree growth. Japanese application 63-226,814 published Sep. 21, 1988 and Canadian application 2,039,894 published Oct. 6, 1992 to Sarma et al disclose an insulation composition comprising a low density PE in admixture with an ethylene-vinyl acetate-vinyl alcohol copolymer as a possible water tree retardant composition.
U.S. Pat. No. 5,719,218 to Sarma proposes for improved water tree resistance for an electrically insulating cross-linked polyethylene composition for use in high voltage electrical cables, the cross-linked polyethylene being obtained by cross-linking a composition consisting essentially of 98% of a low density, peroxide cross-linkable polyethylene, 1-2% of a terpolymer of ethylene, vinyl acetate and vinyl alcohol and at least 0.15% of a sterically hindered amine stabilizer. Commercial acceptance of this formulation has been limited.
Polymers containing peroxides are vulnerable to scorch, i.e., premature cross-linking occurring during the polymer extrusion process. Scorch causes the formation of discolored gel-like particles in the resin and leads to an undesired build up of extruder pressure during extrusion. A good stabilizer package for peroxide cross-linked polyethylene for medium and high voltage cable insulation should protect the polymer against scorch during cable extrusion and provide long term stability after the cable has been produced. Additionally, the cable quality would be negatively affected.
Consequently, a suitable stabilizer system should provide low scorch. In addition to protection from scorch, the stabilizer system has an additional function. After the cable is produced, it is in service for an extended period of time (service life; long term stability). Often, the service life exceeds the intrinsic maximum stability of the polymer. Consequently, stabilizers need to be added in order to assure a suitable service life. During the cross-linking step, the interaction of the stabilizer with the peroxide should be as low as possible to ensure an optimum cross-link density resulting in optimal mechanical properties. Cross-linking assists the polymer in meeting mechanical and physical requirements, such as improved thermal aging and reduced deformation under pressure. Consequently, the stabilizer system, while suppressing scorch during the compounding step (and counteracting the effect of peroxides), should also have as few interactions as possible with the peroxide in later stage of the cable manufacturing process. An excess of organic peroxide may be used to achieve the desired level of cure, but, as described in EP 1088851, this leads to a problem known as sweat out. Sweat out dust is an explosion hazard, may foul filters, and causes slippage and instability in the extrusion process.
Other properties, such as the solubility of the antioxidant in the polymer matrix, are also important. A high solubility of the antioxidants ensures a low level of blooming. Blooming may result in the generation of dust on the pellets, which can lead to health and environmental concerns. Additionally, additives that bloomed to the surface might physically be lost and become unavailable in the polymer matrix for their intended purpose. Consequently, a suitable stabilizer package should have sufficient solubility with the polymer matrix. Further, a low enough melting point is required. A low melting point ensures a good dispersion of the antioxidant in the polymer matrix. Insufficient dispersion leads to decreased performance of the additive in the polymer matrix. An additive with a melting point above the maximum processing temperature of the polymer (as determined by the peroxide) would result in a very poor dispersion of the additive in the polymer matrix. This is considered a substantial drawback. The most appropriate way to incorporate additives into the polymer would be in a liquid form. While the stabilizer system does not necessarily need to be a liquid at room temperature, it needs to melt at a low enough temperature to be easily filtered and added to the polymer in a liquid form. A liquid addition will have the further advantage in that the additive can be filtered, thereby increasing cleanliness. Increased cleanliness of the additive will further improve the cable quality. Consequently, it is desirable that the stabilizing system have a sufficiently low melting temperature and desired properties.
U.S. Pat. No. 3,954,907 discloses that vulcanizable ethylene polymer-based compositions, which are susceptible to scorching when processed at elevated temperatures, prior to vulcanization, and in the presence of certain organic peroxide compounds, can be protected against such scorching by the incorporation therein of monomeric vinyl compounds having a defined structure.
U.S. Pat. No. 5,530,072 discloses a process that is said to improve the modification efficiency of peroxides through the proper selection of anti-oxidant additives and control of the extrusion environment.
U.S. Pat. No. 6,103,374 (EP 0965999 A1) discloses a composition comprising: (a) polyolefin; (b) as a scorch inhibitor, 4,4′-thiobis(2-methyl-6-t-butyl phenol); 2j2′-thiobis(6-t-butyl-4-methylphenol); or mixtures thereof; (c) hydroquinone; a substituted hydroquinon; or mixtures thereof in an amount sufficient to control color formation; and (d) an organic peroxide.
U.S. Pat. No. 6,180,231 (EP 1041582) discloses a composition comprising: (a) polyethylene; (b) as a first scorch inhibitor, a substituted hydroquinone or 4,4-thiobis(2-t-butyl-5-methyl phenol); (c) as a second scorch inhibitor, distearyl disulfide; and (d) an organic peroxide. U.S. Pat. No. 6,180,706 (EP 0965998 A1) discloses a composition comprising: (a) a low density homopolymer of ethylene prepared by a high pressure process; (b) a scorch inhibitor selected from the group consisting of a substituted hydroquinone; 4,4′-thiobis(2-methyl-6-t-butylphenol); 2,2′-thiobis(6-t-butyl-4-methylphenol); and 4,4′-thiobis(2-t-butyl-5-methylphenol) in an amount of about 0.02 to about 0.07 part by weight of scorch inhibitor per 100 parts by weight of homopolymer; (c) a cure booster; and (d) an organic peroxide.
U.S. Pat. No. 6,187,858 discloses a composition comprising: (a) polyethylene; (b) as a first antioxidant, a thiobisphenol; (c) as a second antioxidant, a compound containing 3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate in the molecule; (d) as a third antioxidant, distearyl thiodipropionate; and (e) an organic peroxide, with the proviso that each antioxidant is present in an amount of about 0.01 to the about 0.2 part by weight and the organic peroxide is present in an amount of about 0.5 to about 3 parts by weight, all per 100 parts by weight of polyethylene.
U.S. Pat. No. 6,191,230 discloses a masterbatch composition comprising: (a) a copolymer of ethylene and 1-octene prepared with a metallocene catalyst; (b) a scorch inhibitor of a substituted hydroquinone; 4,4′-thiobis(2-methyl-6-t-butylphenol); 4,4′-thiobis(2-t-butyl-5-methylphenol); or mixtures thereof; (c) a cure booster, triallyl trimellitate; 3,9-divinyl-2,4,8,10-tetra-oxaspiro[5.5]undecane; triallylcyanurate; triallyl isocyanurate; or mixtures thereof; and (d) an organic peroxide.
U.S. Pat. No. 6,869,995 discloses a composition comprising: (i) polyethylene, and, based on 100 parts by weight of component (i), (ii) about 0.3 to about 0.6 part by weight of 4,4′-tbiobis(2-inethyl-6-t-butylphenol); 4,4′-thiobis(2-t-butyl-5-methylphenol); 2,2′-thiobis(6-t-butyl-4-methylphenol); or a mixture of said compounds, and (iii) about 0.4 to about 1 part by weight of a polyethylene glycol having a molecular weight in the range of about 1,000 to about 100,000.
U.S. Published Patent Application No. 2005/0148715 discloses a process for preparing a composition comprising the step of selecting a composition for preparing a moldable, test plaque having (1) a MDR tsl at 150 degrees Celsius of at least about 20, (2) a MDR tsl at 140 degrees Celsius of at least about 50, (3) a retention of tensile strength of at least about 75% after two weeks of aging at 150 degrees Celsius, (4) a retention of elongation of at least about 75% after two weeks of aging at 150 degrees Celsius, (5) water tree resistance less than about 45%, and (6) sweatout of less than about 100 ppm of the thiobis phenolic antioxidant and (b) imparting water tree resistance to the insulation of cables, the composition comprising: (i) polyethylene, and based on 100 parts by weight of component (i), (ii) about 0.3 to about 0.6 part by weight of a thiobis phenolic antioxidant selected from the group consisting of 4,4′-thiobis(2-methyl-6-t-butylphenol); 4,4′-thiobis(2-t-butyl-5-methylphenol); 2,2′-thiobis(6-t-butyl-4-methylphenol); or a mixture of said compounds; and (iii) about 0.4 to about 1 part by weight of a polyethylene glycol having a molecular weight in the range of about 1000 to about 100,000.
EP 1074580 discloses the use of [1,3,5-tris(4-tert-butyl-3-hydroxy-2,6-dimethylbenzyl)-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione] as a scorch inhibitor in the technical field of preparation of cable insulation, semi-conductive shields, and jackets. EP 1088851 discloses the use of α-tocopherol as a scorch inhibitor.
EP 1249845 discloses the use of 2,4-bis(n-octylthiomethyl)-6-methylphenol as an antioxidant for a peroxide crosslinked polyethylene composition used as insulating material for medium and high voltage cables. EP 1249845 also discloses the combination of: a polyethylene; a scorch inhibitor having a melting point below 50° C. at atmospheric pressure; and an organic peroxide. The use of 4,6-bis(octylthiomethyl)o-cresol, as a scorch inhibitor is disclosed along with other structurally related compounds. JP 57-126833 discloses related compounds.
WO 00/02207 discloses peroxide cross-linked polyethylene as an insulating layer for wire and cable purposes that can be stabilized by a two component system based on 2,2′thiodiethylene bis[3(3,5-di-butyl-4-hydroxyphenyl)propionate] (IT) and distearyl 3,3′-thiopropionate (JE), usually at a total loading of about 0.4% total in a 1:1 ratio. It is also disclosed that a single stabilizer approach can be used, more particularly one with combined phenol and sulfur functionality, such as 4,4′-thiobis(2-t-butyl-5-methylphenol).
The use of antioxidant combinations is possible, but only a few of these combinations can meet the desired combination of properties that are required for an insulating material for medium voltage and high voltage power cable comprising, good anti-scorch, limited interaction with the peroxide during cross-linking, good long term stability, good solubility, a low melting point, and good color.
A good overview of the various polyethylene types is given in “Handbook of Polyethylene” by A. J. Peacock (Marcel Dekker Publishers, 2000). A more specific description of suitable polyethylenes is given in U.S. Published Patent Application No. 2005/0148715 A1 (page 2 paragraph [0017] to page 3 paragraph [0023]).
Therefore, a need exists in the electrical cable industry for an additive system that improves the tree resistance performance of polyolefin polymers as an electrical insulation composition.